Fixed warnings about an ambigous ampersand in serializer.rb

I've been seeing this warning:

    oat/lib/oat/serializer.rb:54: warning: `&' interpreted as argument prefix

You can reproduce this by running rspec with the "-w" flag (before this
change). For some reason I was seeing it every time I tested an oat
serializer in my own app.

This fixes it by making the ampersand unambiguous.